The present invention relates to a fast neutron nuclear reactor of the type in which the reactor core is housed in a vessel, called the main vessel filled with liquid metal and sealed by a slab. More specifically, the invention relates to the supporting of the main vessel and the sealing slab by the upper part of the vessel shaft formed in the concrete enclosure for receiving the main vessel, in the case of a reactor of the aforementioned type.
FIG. 1 is a diagrammatic cross-sectional view of a known, integrated fast neutron reactor. It is possible to see the concrete enclosure 10 defining the vessel shaft 12, in the upper part of which is hung the main vessel 14 containing the reactor core 16. The latter rests on a supply support 18, which itself rests on the bottom of vessel 14 via flooring 20.
The upper end of the main vessel 14 is sealed by a sealing slab 22 and contains the primary liquid metal 24, generally sodium. An inner vessel 26 separates the sodium 24 in vessel 14 into two separate volumes. Thus, the inner vessel 26 respectively defines a hot collector 24a and a cold collector 24b.
In its peripheral part, slab 22 supports a certain number of heat exchangers 28 and pumps 30. Under the action of pumps 30, the hot sodium leaving the reactor core 16 traverses the hot collector 24a prior to entering exchangers 28 through inlets 28a. The heat carried by the primary fluid is then transmitted to the secondary fluid. The cooled primary sodium leaves the exchangers through outlets 28b issuing into the cold collector 24b. It is then taken up by pumps 30 and is delivered to the supply support 18 of core 16 by pipes 32. Generally, the main vessel 14 is duplicated by a safety vessel 34, which is also suspended on the upper part of the vessel shaft.
Fast neutron nuclear reactors of the loop type are distinguished from fast neutron reactors of the integrated type by the fact that the exchangers and optionally the pumps are no longer located within the main vessel and are instead positioned outside the latter. However, the operation of the reactor and the problems connected with the supporting of the vessel and the slab are the same. Therefore, the present invention can apply either to integrated or loop-type fast neutron reactors.
FIG. 2 shows on a larger scale and in greater detail the supporting of vessels 14 and 34, as well as slab 22 by the upper part of the vessel shaft in the reactor of FIG. 1. It is clearly visible that the main vessel 14 and sealing slab 22 are suspended in the upper part of the vessel shaft by a common support ferrule 36, which forms both the outer ferrule of the slab and the upper part of the main vessel 14.
In this configuration, the attachment point 40 of the lower base plate 42 of the slab to the upper part of vessel 14 must be positioned at a certain distance above the internal neutral gas atmosphere above the free sodium level of the main vessel 14 and must be thermally insulated from said atmosphere. Thus, if such a precaution is not taken, the upper part of the main vessel would be exposed to unacceptable thermal stresses resulting from the large temperature difference existing between the slab 22, cooled by a circulation of fluid diagrammatically represented by the arrows in FIG. 2, and the internal neutral gas atmosphere of the vessel 14. Thus, FIG. 2 shows that it is conventional practice to produce slab 22 in such a way that it has a downwardly projecting portion with respect to its attachment point 40 on the vessel. Thus, between vessel 14 and the lower base plate 42 of the slab, there is an annular zone 43, in which is placed a heat insulating material 44.
Moreover, it is known that the thickness of slab 22 is determined, particularly with regards to its concrete filling, by a compromise between its neutron protection function and the criteria linked with its mechanical behaviour and cost.
Thus, FIG. 2 shows that, according to the prior art, this compromise makes it necessary to place the upper base plate 46 of slab 22 at a lower level than the upper face of concrete enclosure 10. Thus, for a given dimensioning of the reactor block, if the thickness of the slab is approximately 2700 mm, this displacement reaches approximately 850 mm.
As a result of this lowering of the upper level of the slab relative to the concrete enclosure surrounding it, any sodium leak occurring at the passage through the slab may lead to a sheet-like sodium fire thereon. The consequences of such a fire can be relatively serious, so that in existing structures, it is necessary to provide protection elements, such as one or more sodium retention tanks.
Moreover, the downward displacement of the slab 22 relative to the upper face of the concrete enclosure 10 increases by the same amount the vertical dimensions of certain of the reactor components. In particular, the heights of the concrete enclosure 10, the safety vessel 34 and its thermal insulation are increased by this displacement. Other components, such as exchangers and handling locks consequently also have an increased height at the point of traversing the slab. This leads to an increase in the cost of the reactor.
The present invention specifically relates to a fast neutron nuclear reactor, in which the main vessel and its sealing slab are separately suspended on the vessel shaft. This eliminates any displacement between the upper face of the slab and the upper case of the concrete enclosure surrounding it. This prevents any risk of sheet-like sodium fire on the slab, without it being necessary to use costly ancillary means. Moreover, the height of a certain number of reactor components is reduced by the height of the displacement of the slab relative to the upper face of the concrete enclosure which, in the aforementioned example, represents approximately 850 mm, which leads to a substantial reduction in the costs of the reactor.
Thus, the invention relates to a fast neutron nuclear reactor comprising a liquid metal-filled main vessel containing the reactor core, a sealing slab which seals the vessel and a concrete enclosure defining a vessel shaft in which are located and suspended the main vessel and its sealing slab, wherein the slab and the main vessel are directly suspended on the upper part of the vessel shaft by separate supporting means.
According to a preferred embodiment of the invention, the slab comprises a peripheral ferrule, whose upper end is anchored in the concrete enclosure in the upper part of the vessel wall by anchoring means, and a first horizontal support ring, provided with welded vertical stiffeners and whose inner periphery is welded, during production, in the vicinity of the upper end of the ferrule and whose outer periphery is sealed in the concrete enclosure below the anchoring means, the upper end of the main vessel being welded, following its installation in the vessel shaft, to the first support ring at a given distance from the peripheral ferrule of the slab, the supporting means for the main vessel also comprising a second horizontal support ring provided with vertical stiffeners and whose inner periphery is welded, during production, in the vicinity of the upper end of the main vessel and whose outer periphery is sealed in the concrete enclosure.
When the reactor also comprises in per se known manner a safety vessel surrounding the main vessel, the safety vessel can be directly suspended in the upper part of the vessel shaft by supporting means separated from the supporting means for the main vessel and the supporting means for the slab.
According to a preferred embodiment of the invention, the upper end of the safety vessel is welded, following installation in the vessel shaft, to the second support ring at a given distance from the main vessel, the supporting means for the safety vessel also comprising a third horizontal support ring, provided with vertical stiffeners and whose inner periphery is welded, during manufacture, to the safety vessel, in the vicinity of its upper end, said third support ring being sealed in the concrete enclosure.
According to another appect of the invention, the slab has an upper portion laterally defined by a second ferrule, whose lower end is fixed to the peripheral ferrule below the first support ring, by means of a connecting ring, said ferrules defining between them an annular space housing a thermal insulating means, the upper face of the slab being substantially located in the same horizontal plane as the upper face of the concrete enclosure around the vessel shaft.
In this case, the main vessel defines with the peripheral ferrule of the slab and with the upper end of the safety vessel, two annular spaces which can house heat insulating means, which are relatively tight to sodium leaks and compatible with the said liquid metal.